1. Field of the Invention
This invention relates to a process suitable for forming a deposited film, above all a functional film, particularly an amorphous or crystalline deposited film to be used for semiconductor devices, photosensitive devices for electrophotography, line sensors for image input, image pick-up devices, photovoltaic devices etc.
2. Description of the Prior Art
For example, for formation of an amorphous silicon film, an amorphous germanium film, etc. the vacuum deposition method, the plasma CVD method, the CVD method, the the reactive sputtering method, the ion plating method, the optical CVD method and the like have been practiced, and, in general, the plasma CVD method has widely been used and industrialized.
However, for the deposited film constituted of amorphous silicon, amorphous germanium, etc. there is room for further improvement in overall characteristics including and optical characteristics, fatigue characteristic on repeated use, and resistance to environmental influences, and also in productivity and mass productivity including uniformity and reproducibility.
The reaction process in formation of an amorphous silicon deposited film, an amorphous germanium deposited film, etc. according to the plasma CVD method of the prior art is considerably more complicated than the CVD method of the prior art, and several ambiguities exist in its reaction mechanism. Also, there are involved a large number of parameters for formation of such a deposited film (e.g. substrate temperature, flow rates and ratios of gases introduced, pressure during film formation, high frequency power, electrode structure, structure of reaction vessel, gas discharging speed, plasma generation system, etc.), and the plasma formed by combination of such a large number of parameters may sometimes become unstable to frequently adversely affected the deposited film formed. Besides, the parameters inherent in the device must be chosen for each device, and it has been difficult under the present situation to generalize the production conditions. On the other hand, for exhibiting electrical, optical, photoconductive or mechanical characteristics of an amorphous silicon film, an amorphous germanium film, etc. satisfactory for respective uses, it has been deemed best to form such a film according to the plasma CVD method under the present situation.
However, depending on the applied uses of the deposited film, since it is required to meet sufficiently requirements of enlarged area, uniform film thickness and uniform film quality, and also to attempt mass production with reproducibility by a high speed film formation, enormous equipment capital becomes necessary for mass production devices in formation of amorphous silicon deposited film, amorphous germanium deposited films, etc. according to the plasma CVD method. Also managing the parameters in mass production becomes complicated requiring narrow operating tolerances. These matters, and also subleties in adjustment of the devices, have been pointed out as the problems to be improved in the future. On the other hand, in the conventional CVD method of the prior art, high temperature is required to be used and no deposited film having practical characteristics could be obtained.
As described above, in formation of amorphous silicon films, amorphous germanium films, etc. it has earnestly been desired to develop a formation process which can perform mass production by means of a low cost device while maintaining practical characteristics and uniformity. These concerns are also applicable to other functional films such as silicon nitride films, silicon carbide films, silicon oxide films, etc.